Interpreting Geological Maps When Site Prospecting

[JeepDigger]

Hey all, I am trying to plan a couple little trips around the midwestern US and I have been looking at satallite images online and some geological/topographical maps but I'm not 100% sure what I am looking for. I found a few posts that touched on this briefly when discussing site prospecting but I would like some more info. Any advise or reference material that could help me interpret these maps/pictures and pin down some sites?

[Missourian]

Some pointers I've picked up over the years. You may already be aware of some, if not all....

Escarpments and steep-sided valleys - These indicate where resistant formations outcrop:

Chalk - The stuff really stands out on satellite images. That is, if you're planning a trip out to western Kansas:

Creeks and rivers - check where they curve into a slope:

Active quarries - These appear as white splotches on sat images:

Abandoned quarries - Look for ponds in odd places:

Strip coal mines - Look for the ribbon lakes and water-filled 'corn rows':

Roads - Check where they cross slopes:

Also, road cuts often stand out nicely on Google Maps:

Google Street View has turned out to be extremely useful for my purposes:

[trilobiteruss]

Nice posts Missourian, good presentation, all that is just common knowledge to a geologist like me, but I forget how this is not to folks who did not have a career in this. We used to have a publication on interpreted Top maps that helped a bit but I like your little example photos, they are good examples and should help anyone to get a good start! Well done! Did you have this saved somewhere? I would have had to dig it all up and find good examples like you did.

russ

[Uncle Siphuncle]

great info. allow me to supplement these remarks. in streams i often find myself one formation lower than what is mapped in that area due to downcutting, and the lower formation often extends farther downstream than the mapped contact in that area (dip effects this as well).

in areas where thin formations or interfingering result in lumping of mapped formations, you just have to go, look, observe, and figure it out based on lithology and fauna.... pays to study up on both before hitting the field.

i don't know of any shortcuts in this, but using your mind and senses to solve the riddle is half the fun!

[Missourian]

Thanks, T-russ.

I didn't have this saved and ready to go, but I was pondering starting a thread with some pointers (and hoping others would chime in with some others).

As for the examples, I recalled where they were, and snatched them from Google. I wish I could find more examples in certain areas....

[Missourian]

.... And someone chimed in already. Thanks Danwoer.

[trilobiteruss]

Have to look around and see if I can find some good info on topo and geological maps as well to complete excellent pointers already here...

[trilobiteruss]

OK here is one on reading topographic maps on the Illinois Geological Survey Page:

How to Read Illinois Topographic Maps

It is too big to upload here....

russ

[trilobiteruss]

Here is one nice link on Geological Maps: http://www.nature.nps.gov/geology/usgsnps/gmap/gmap1.html

[Bullsnake]

Excellent thread!!! Thanks, JeepDigger for starting it, and all with the answers/tips!

[JeepDigger]

Awesome responses! Pictures and everything help out alot Missourian as well as the links. I hope people keep posting on this thread. All of us "newbs" need all the help we can get!

[Uncle Siphuncle]

the kühl thing is that your increasing field skill set transfers to wherever you travel, making you an increasingly effective collector in new areas, even out of the state or out of the country. while geology varies with locale, geological principles don't. stick with it and you'll be making high return strikes on sites and formations rarely hunted or discussed, constantly breaking new and productive ground. like many things in life, with perseverence it becomes easier. still, your education will be earned largely by the sweat of your brow. best of luck!

[Missourian]

great info. allow me to supplement these remarks. in streams i often find myself one formation lower than what is mapped in that area due to downcutting, and the lower formation often extends farther downstream than the mapped contact in that area (dip effects this as well).

in areas where thin formations or interfingering result in lumping of mapped formations, you just have to go, look, observe, and figure it out based on lithology and fauna.... pays to study up on both before hitting the field.

i don't know of any shortcuts in this, but using your mind and senses to solve the riddle is half the fun!

You pretty much summed up many of the issues I've had with geo maps.

It's good to remember that the placement of formations is necessarily 'fudged' due to lack of data. (Of course, the lack of data is the reason for having the map in the first place.... ).

In some cases, there may be really thick deposits of glacial till and loess that may or may not be taken into account by the mappers. Entire hills may be mostly loess, with unknown amounts of bedrock beneath. One map in my posession does mark the known and probable loess deposits blanketing much of the area. The bedrock outcroppings shown are limited to measured sections in stream valleys and on a few slopes.

Also, be aware that mistakes can and are made on maps. In a local example, the map stated that a certain limestone formation was present in outcrops along a creek. When I showed up, they turned out to be of the overlying formation. I'm sure the mapper, having seen the lower formation in a nearby stream, thought some knobs of limestone in the creek bed were the same. Well, he/she didn't carefully check the context of the adjacent strata....

In an even worse case, just upstream, he/she 'created' a fault out of whole cloth. What he/she thought was a "normal fault with 60 feet of displacement" was actually no more than a couple large blocks of limestone that had slid down from a ledge high up on the slope:

In defense of the professor emeritus who published the map, he certainly would not have made those errors. One of his assistants doing the legwork must have been having a bad day. But then again, the professor should have followed up on their work before publication, especially regarding the fault....

Edited September 6, 2012 by Missourian

[JeepDigger]

Excellent pointer about the faults in the maps. One of the ones I was looking at online last night in the Kansas chalk hills showed a steep wall along a creek that looked perfect but thanks to google maps I could see the bend in the creek a few hundred yards from the highway and the "steep creek cut" turned out to be 5-7 ft tall! Still looks promising but not the jackpot I was hoping for.

[Missourian]

Chalk is a whole different ballgame than the Pennsylvanian-Permian rocks in our area. Gentler slopes are what you want. It's nice to walk rather than climb. The fossils that do weather out stay in place and don't wash downstream. And when you do find something big, you don't have to worry about removing a whole lot of overburden.

Edited September 6, 2012 by Missourian

[Auspex]

The big, historic breakthrough in geology was to use fossils to correlate the strata. It was not meant, and is still not meant, to help folks find fossils; it is meant to make sense of the extent and distribution of all the other valuable resources. Condense this idea into a mantra, to be repeated to calming effect when the fossils "aren't where they should be".

[trilobiteruss]

Missourian, as a person who mapped for 20 plus years in southern and western Illinois I probably would not be as harsh as you about all geological maps. yes there are mistakes or outcrops that are partially covered and inferences have to be made. I do know of some mappers who do not do a good job, but most mappers I have worked with do the best job possible. The ones we did are usually accurate to 5-10 feet at worse. as it is hard to get any more precise in plotting on the scale of contours of a 7.5 minute topographic map. So if you are expecting plus/mius 1 foot then you will be disappointed for sure. They are another tool but if you expect too much of them beyond the 5 foot plus or minus of 7.5 quads then you are expecting more than any mapper could deliver. As a mapper understanding those few limitations I find maps done in other states to be very usefull in locating formations of interest, and looking at the description of the units combined with contours on the maps has allowed me to do good collecting in other states.

Just two cents from an old mapper ;)

[trilobiteruss]

Also in our projects in southern Illinois we did two types of maps, one was a surficial map by our bedrock and glacial people where there were such. But we then would also do a bedrock map that was just bedrock, I have seen both types, one a map showing the surface as it actually is, mostly not bedrock and where the outcrops are. Southern Illinois was used more though because much of shawnee forest area was unglaciated, so primarily bedrock maps could be done, and in transitional areas we did both. The surficial map was done primarily by our glacial folk so the bedrock exposures they found were generalized.

At times we and others will take the contours and dash where inferred and partially covered, and dotted where totally inferred from ravines and subsurface data, so a good map will use those techniques where there is a thick unconsolidated loess or glacial drift. But sometimes in some states mappers may not do that but a good geological map will.

[Missourian]

Perhaps I used too broad of a brush in describing errors on geo maps. I was just disappointed with my local map after looking so forward to receiving it. The stratigraphic section desription actually stated that such-and-such formation was exposed in the creek bed, when it was actually the one above it, and the 'structural geology' description mentioned the 'fault'. Otherwise, though, the map is excellent.

I understand and don't mind that there will be inevitable ballpark estimates. These maps are particularly useful if your goal is to greatly narrow down an area to search for a certain layer, rather than find a spot down to the foot.

Edited September 7, 2012 by Missourian

[trilobiteruss]

Perhaps I used too broad of a brush in describing errors on geo maps. I was just disappointed with my local map after looking so forward to receiving it. The stratigraphic section desription actually stated that such-and-such formation was exposed in the creek bed, when it was actually the one above it, and the 'structural geology' description mentioned the 'fault'. Otherwise, though, the map is excellent.

I understand and don't mind that there will be inevitable ballpark estimates. These maps are particularly useful if your goal is to greatly narrow down an area to search for a certain layer, rather than find a spot down to the foot.

Yep, just wanted to not discourage people from using maps, maybe understanding a bit the limitations of what you can and cannot map and how things are done. There are maps that are not that good or two general. I know a mapper who will not infer from ravine to ravine with covered intervals, but you can find float between that makes your inferrence more reliable. We redid his maps rather than let them stand

[erose]

Trilobiteruss mentions one thing that I find very valuable but is often ignored by new collectors. That is the written descriptions of the formations. The maps are pretty and always helpful but it will be the written description that accompanies the map that is most useful. Some maps have the information written on the sheet itself and others will have an additional small booklet. If you have trouble understanding the descriptions (some are totally geo jargon) get yourself a geological dictionary. Some of those big words are actually really easy to understand, like argillaceous = contains clay or arenaceous = sand sized particles. Also they can tell you what the fresh stone looks like versus the weathered.

[squalicorax]

Do not forget about http://tin.er.usgs.gov/geology/state/ to download KMZ files of your state and import them into Google Earth. Valuble tool for determining the stratigraphic information of a located outcrop using the well documented processes above.